The subject matter disclosed herein relates to arc flash detection and mitigation technologies, and particularly relates to detection devices for this purpose.
Electric power circuits and switchgear have conductors separated by insulation. Air space often serves as part or all of this insulation in some areas. If the conductors are too close to each other or voltage exceeds the insulation properties, an arc can occur between conductors. Air or any insulation (gas or solid dielectrics) between conductors can become ionized, making it conductive, which enables arcing. Arc temperature can reach as high as 20,000° C., vaporizing conductors and adjacent materials, and releasing significant energy.
Arc flash is the result of a rapid energy release due to an arcing fault between phase-phase, phase-neutral, or phase-ground. An arc flash can produce high heat, intense light, pressure waves, and sound/shock waves. However, the arc fault current is usually much less than a short circuit current, and hence delayed or no tripping of circuit breakers is expected unless the breakers are selected to handle an arc fault condition. Agencies and standards such as the National Environmental Policy Act (NEPA), Occupational Safety and Health Administration (OSHA), and Institute of Electrical and Electronics Engineers (IEEE) regulate arc flash issues through personal protective clothing and equipment, but there is no device established by regulation to eliminate arc flash.
Standard fuses and circuit breakers typically do not react quickly enough to an arc flash. To provide a safety mechanism with sufficiently rapid response, there are common arc flash mitigation devices, such as the electrical “crowbar,” utilize mechanical and/or electromechanical processes. For example, an electrical crowbar is a protection device that intentionally shorts an electrical circuit and thus diverts the electrical energy away from the arc flash. The intentional 3-phase short circuit fault thus created is then cleared by tripping a fuse or circuit breaker, and shutting down the power. However, such intentional short circuits may allow significant levels of current resulting from the intentional short-circuit. Regardless of the arc mitigation mechanism, there is a need in the art for an arc flash detection arrangement that can distinguish between arc flash events from normal or expected operation.
Radiation sensors can be used to detect the presence of radiation in various electromagnetic spectrum regimes. However, such sensors tend to be sensitive to relatively low light levels and so, when employed to detect radiation associated with an arc flash event, will tend to detect non-arc-flash radiation, so called “nuisance light,” such as sunlight, flashlights, room lights, and the like, which vary in intensity from about 500 lux (flashlight) to about 2,000 lux (commercial space lighting) to about 80,000 lux (direct sunlight).
Lux is a desired measurement as lux is proportional to measuring meters for distance. The further away the light source is from a sensor, the more significant the intensity diminishes. For example, if it is assumed impendent light was impinged on a volume of a cube for every meter out the distance extended, the intensity of the light would drop off by a square factor of the distance. Atmospheric and obstacles could change that estimation. Some light sources such as flashlights dissipate relatively quickly, while sunlight has little change with distance. By using lux as the value of light measured at a sensor, the use of a light meter (e.g., lux meter) to calibrate and verify sensor outputs is possible.
In addition, most sensors become saturated well below the level at which an arc flash event would be emitting. For example, a typical arc flash event will produce radiation in the visible spectrum with luminous flux on the order of 100,000 lux at 3-4 feet from the arc flash event, while most point light sensors saturate at 700 lux or less. Thus, there is a need for a radiation sensor that can discriminate between nuisance light and radiation produced by an arc flash event while maintaining sensitivity in the required range.